U.S. patent number 6,819,872 [Application Number 10/431,752] was granted by the patent office on 2004-11-16 for micro-optic delay element for use in a time division multiplexed system.
This patent grant is currently assigned to JDS Uniphase Corporation. Invention is credited to Yihao Cheng, Paul E. Dunn, Mark Farries, Andrew Finch, Timothy C. Munks.
United States Patent |
6,819,872 |
Farries , et al. |
November 16, 2004 |
Micro-optic delay element for use in a time division multiplexed
system
Abstract
A micro-optical delay element for a time-division multiplexing
scheme is disclosed wherein two light beams are provided to a beam
splitter/combiner (BS/C) in the absence of optical fibre. At least
one beam exiting a modulator is collimated and reaches the (BS/C)
unguided as a substantially collimated beam. This obviates a
requirement for polarization controllers and polarization
maintaining optical fiber
Inventors: |
Farries; Mark (Exeter,
GB), Cheng; Yihao (Ottawa, CA), Munks;
Timothy C. (North Granby, CT), Dunn; Paul E. (North
Granby, CT), Finch; Andrew (Avon, CT) |
Assignee: |
JDS Uniphase Corporation (San
Jose, CA)
|
Family
ID: |
28794280 |
Appl.
No.: |
10/431,752 |
Filed: |
May 8, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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342154 |
Jun 23, 1999 |
6607313 |
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Current U.S.
Class: |
398/102; 398/147;
398/152; 398/161; 398/182; 398/183; 398/184; 398/65; 398/79 |
Current CPC
Class: |
G02B
6/2861 (20130101); H04J 14/06 (20130101); H04J
14/08 (20130101) |
Current International
Class: |
G02B
6/28 (20060101); H04J 14/06 (20060101); H04J
14/08 (20060101); H04J 014/08 () |
Field of
Search: |
;398/102,152,65,182,183,184,161,147,79 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Y Miyamoto, K. Yonenaga, et al., "104-Tbit/s DWDM Transmission
Experiment Based on Alternate-polarization 80-Gbit/s OTDM Signals,"
ECOC '98, Madrid, Spain, Sep. 20-24, 1998, p. 55 and 57. .
Single Mode Fibre Fractional Wave Devices and Polarisation
Controllers, Electronics Letters, Sep. 25, 1980, vol. 16. No. 20,
pp. 778-780..
|
Primary Examiner: Phan; Hanh
Attorney, Agent or Firm: Allen, Dyer, Doppelt, Milbrath
& Gilchrist, P.A.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is a continuation-in-part of co-pending
U.S. patent application Ser. No. 09/342,154, filed Jun. 23, 1999
now U.S. Pat. No. 6,607,313, by Farries et al., entitled:
"MICRO-OPTIC DELAY ELEMENT FOR USE IN A POLARIZATION MULTIPLEXED
SYSTEM".
The present application claims priority from U.S. provisional
application Ser. No. 60/378,777 filed May 8, 2002, by Munks et al.
Claims
What we claim is:
1. A circuit for receiving two streams of data and for polarization
time-division multiplexing the two streams of data onto a single
waveguide such that one of the data streams is delayed by a time
T.sub.d from the other data stream, comprising: a first modulator
having an input port and an output port for modulating input light
and for providing a first modulated data stream; a second modulator
having an input port and an output port for modulating input light
and for providing a second modulated data stream; a first lens for
collimating light provided by the first modulator; a second lens
for collimating light provided by second modulator, the first and
second lenses each for providing a substantially collimated
substantially unguided beam of light to at least another component;
a polarization beam splitter/combiner having two input ports at one
end optically coupled to receive the substantially collimated,
substantially unguided beams of light, said polarization beam
splitter/combiner having a combining port at another end for
combining the data streams, wherein optical path from one input
port of the splitter/combiner to the combining port is different
than optical path from the other input port of the
splitter/combiner to the combining port such that one data stream
is delayed by a time T.sub.d from the other data stream, whereby
the block has both a light combining function and a time delay
function, light traversing parallel paths from the first and second
lenses respectively to the polarization beam splitter/combiner
being substantially unguided, so that light traversing at least one
of said parallel paths will have a polarization state which is
substantially unchanged.
2. A circuit as defined in claim 1 further comprising means for
rotating the polarization of one of the two modulated data streams
optically coupled with one of the first and second modulators.
3. An optical circuit as defined in claim 2, wherein the first and
second modulators, the means for rotating polarization, and the
polarization beam splitter/combiner are all optically coupled
without optical fibres therebetween, and wherein the first and
second lenses are GRIN lenses.
4. An optical circuit as defined in claim 3, wherein the
polarization beam splitter/combiner is a birefringent crystal being
of suitable length for providing a path length difference between
each of the at least two input ports and the output port to provide
a time delay of T.sub.d at the output port between the two data
streams.
5. An optical circuit as defined in claim 3, wherein the
polarization beam splitter/combiner comprises polarization beam
splitting cubes being of suitable dimensions for providing a path
length difference between each of the at least two input ports and
the output port to provide a time delay of T.sub.d at the output
port between the two data streams.
6. An optical circuit as defined in claim 3, wherein the
polarization beam splitter/combiner is a birefringent crystal being
of suitable length for providing a path length difference between
each of the at least two input ports and the output port to provide
a time delay of T.sub.d at the output port between the two data
streams in combination with a delay element disposed in the optical
path between one of the modulators and a longest path from one of
the input ports of the birefringent crystal to its output port.
7. A circuit as defined in claim 1 wherein a geometrical distance
from each modulator output port to respective optically coupled
input ports of the polarization beam splitter/combiner is
substantially the same, and wherein the optical path lengths from
the first and second modulator output ports to the combining port
of the polarization beam splitter/combiner are different to effect
the time delay t.sub.d.
8. The circuit as defined in claim 1, further comprising a spacer
element disposed in one of the parallel paths with a means for
rotating polarization disposed in the other path, the spacer
element for equalizing a geometric distance between parallel paths
from the output ports of the modulators to the input ports of the
polarization beam splitter/combiner.
9. The circuit as defined in claim 1, wherein the lenses are GRIN
lenses, the circuit further comprising a spacer element in at least
one of the parallel paths having a refractive index substantially
higher than an average refractive index of either of the GRIN
lenses for substantially delaying a beam of light propagating
therethrough.
10. The circuit as defined in claim 9, wherein the spacer element
is made of silicon.
11. A fibreless optical circuit for receiving two streams of
modulated data and for polarization multiplexing the two data
streams onto a single waveguide, comprising: a modulator module for
independently, and in parallel, modulating optical signals and for
providing two data streams; a birefringent crystal having at least
two input ports at an end thereof disposed to receive the two
modulated data streams having different polarization states, the
birefringent crystal having an output port disposed at an opposite
end thereof to receive and combine the two modulated data streams
into a single multiplexed data stream, the birefringent crystal
being of suitable dimensions to provide time division polarization
multiplexing, the circuit being fibreless such that there is an
absence of optical fibre between the modulator module and the
birefringent crystal for coupling light therebetween.
12. A circuit for receiving two streams of modulated data and for
polarization and time-division multiplexing the two streams onto a
single waveguide, comprising: a light source for providing a
primary signal; a first and a second modulator for independently,
and in parallel, modulating portions of the primary signal, the
first and second modulators for providing two data streams; means
for operating on at least one of the two data streams and for
providing different polarization states between the two data
streams; and, a birefringent crystal having at least two input
ports at an end thereof disposed to receive substantially unguided
collimated light from first and second modulators, said unguided
collimated light being in the form of two modulated data streams
having different polarization states, the birefringent crystal
having an output port disposed at an opposite end thereof to
receive and combine the two modulated data streams into a single
time-interleaved data stream, the birefringent crystal being
optically coupled with the modulators in the absence of optical
fiber therebetween, the birefringent crystal being of suitable
shape and dimensions to provide a required optical path length
difference between the two modulated data streams passing
therethrough to time multiplex the two data steams into the single
data stream whereby birefringent crystal has both a combining
function and a time delay function.
13. A method of multiplexing optical signals onto an output port,
comprising the steps of: providing two modulated polarized optical
signals having a polarization difference between the two modulated
signals of substantially 90 degrees; passing one of the two
modulated signals along a first path in a birefringent crystal; and
passing another of the two modulated signals along a second path
intersecting the first path at the output port of the birefringent
crystal, whereby the modulated signals are combined, wherein the
modulated signals are passed to the birefringent crystal in the
absence of optical fiber, and wherein the first path and the second
path are of different length, whereby the birefringent crystal
combines the two modulated signals into a single time-interleaved
data stream.
14. A circuit for receiving two streams of data and for
time-division multiplexing and interleaving the two streams onto a
single waveguide, comprising: a) modulation means for providing
first and second modulated data streams in the form of a first beam
and a second beam; b) means optically coupled with the modulation
means for routing and combining the first and second beams in an
unguided manner, such that beams of light launched into said means
are unguided as they propagate therethrough in the absence of
waveguides, said means for routing and combining having at least
two input ports optically coupled to receive the first and second
beams and having an output port to on which to combine the two
beams into a single time-interleaved data stream, the means for
routing and combining for providing an optical path length
difference along two paths between each of the at least two input
ports and the output port for light launched into the at least two
input ports on route to the output port to provide a required time
delay at the output port between the two data streams such that the
data within the data streams is time interleaved having a bit
period Dt.
15. A circuit as defined in claim 14, further comprising a voltage
controlled attenuator in each waveguide and a means of monitoring
the optical power in each of the two beams such that their relative
power levels can be adjusted.
16. A circuit as defined in claim 14 wherein the two streams of
data are orthogonally polarized, and wherein the means for routing
and combining is polarization dependent for combining the two
orthogonally polarized data streams in a polarization dependent
manner, and wherein the bit period Dt is predetermined.
17. A circuit as defined in claim 14, further comprising a means
for relatively orienting the polarization of the two data streams
such that they are orthogonally polarized.
18. A circuit as defined in claim 17, wherein the means for routing
and combining is polarization dependent for combining the two
orthogonally polarized data streams in a polarization dependent
manner.
19. A circuit as defined in claim 18, wherein the means for routing
and combining is a birefringent crystal.
20. A circuit as defined in claim 18, wherein the means for routing
and combining includes a polarization beam combiner.
21. A circuit for receiving two orthogonally polarized streams of
data and for polarization and time-division multiplexing the two
streams onto a single waveguide, comprising: polarization dependent
means having: first and second input ports at an end thereof; an
output port optically coupled with the first and second ports; a
first unguided optical path and a second unguided optical path
disposed between the first and second input ports respectively and
the output port, for receiving the two orthogonally polarized data
streams and for carrying said data streams to said output port in
an unguided manner, the first and second unguided paths being of a
different optical path length which differ by an optical path
length DnL, wherein the length dnL, is selected to provide a
required relative time delay between the two data streams as they
pass along the first and second unguided paths from the first and
second input ports to the output port such that the data streams
become a single time-interleaved data stream having a predetermined
bit period Dt.
22. A circuit as defined in claim 21, wherein Dt=DnL/c.
23. A circuit as defined in claim 22, further comprising a voltage
controlled attenuator in each waveguide and a means of monitoring
the optical power in each of the two beams such that their relative
power levels can be adjusted.
Description
FIELD OF THE INVENTION
This invention relates generally to optical fiber communications
and in particular to multiplexed communications that uses
time-division multiplexing.
BACKGROUND OF THE INVENTION
High-speed time-division-multiplexing (TDM) is a very attractive
way of enhancing the spectrum efficiency of a large-capacity
wavelength-division multiplexing (WDM) system. One common
architecture employs two modulators having a same bit rate, wherein
two separately modulated streams of data bits are combined into a
high-speed single serial stream of data bits. Instead of providing
a single higher-cost higher-speed modulator capable of providing
modulation at a frequency of n Hz, two modulators having a
frequency of n/2 Hz are provided and their outputs are
time-interleaved providing a signal having a frequency of n Hz.
However, one drawback to such a scheme, particularly in high-speed
dense systems is that pulses from adjacent time slots spread and
partially overlap one another and detection errors sometimes occur
at a receiver end.
Such systems typically use lengths of optical fibre or other delay
means to provide a required optical path length difference between
two paths such that a predetermined delay between two data streams
is provided to achieve bit interleaving.
One remedy for this is provided by an enhanced TDM system wherein
adjacent interleaved pulses are distinguishable as they are
orthogonally polarized. Such a scheme is described in a paper
entitled 1.04-Tbit/s SWDM Transmission Experiment Based on
Alternate-Polarization 80-Gbit/s OTDM Signals, by Yutaka Miyamoto
et al., published in ECOC'98 Sep. 20-24, 1998 Madrid, Spain. In
this paper alternate-polarization optical-TDM is described to
increase the bit rate while keeping the signal spectrum from
broadening. Here two modulated signals are time-division
multiplexed with additional enhancement being achieved by
polarization multiplexing of the two interleaved TDM streams.
Another system using enhanced polarization optical TDM is described
and illustrated in U.S. Pat. No. 5,111,322 in the name of Bergano
et al, entitled Polarization Multiplexing Device with Solitons and
Method Using Same, incorporated herein by reference. In this
patent, a transmission system's capacity is increased by using a
combination of polarization and time-division multiplexing. More
specifically, two streams of differently (preferably orthogonally)
polarized solitons are interleaved (time-division-multiplexed) at a
transmitter, and later separated at the receiver to recover both
data streams.
The multiplexing of 2 channels of 2.5 Gbits/s each, into a single 5
Gbits/s channel, and the corresponding demultiplexing at the
receiving end, is described in conjunction with the multiplexor of
FIG. 2 in prior art U.S. Pat. No. 5,111,322.
In FIG. 2 the signal source for the two channels is a single,
mode-locked laser 201, producing about 35-50 ps wide soliton pulses
at a 2.5 GHz rate. Its output is split into two soliton pulse
streams having essentially orthogonal polarizations, in a splitter
202, and each half separately modulated (with different information
bearing signals labeled Data 1 and Data 2) in modulators 205 and
206. Modulator 205 receives a first information bearing signal or
data stream on line 207, while modulator 206 receives a second data
stream on line 208. The two soliton pulse streams then recombine in
a splitter 210, but only after one of the pulse streams is delayed
by one-half of the 2.5 Gbit/s bit period in an adjustable delay
line 209 so that the two pulse streams are interleaved in time.
A few practical details concerning the apparatus of FIG. 2 are in
order here. The modulators 205, 206 should preferably be of the
LiNbO.sub.3, balanced Mach-Zehnder type, as those produce virtually
no chirping of the soliton pulses, and have an adequate on-off
ratio (.about.20 dB). The required linear polarizations at the
inputs to modulators 205, 206, and for the polarization
multiplexing itself, can either be maintained through the use of
(linear) polarization-preserving fiber throughout the multiplexor,
or through the use of polarization controllers, such as controllers
211-214, both before and after modulators 205, 206 as shown in FIG.
2. Polarization controllers 211-214 may be arranged as described in
an article by H. C. Levevre, "Single-Mode Fiber Fractional Wave
Devices and Polarization Controllers", Electronics Letters, Vol.
16, p. 778, 1980. For the temporal interleaving of the two soliton
pulse streams, it is necessary to make precise adjustment of the
relative lengths of the two arms of the multiplexor. This can be
done with adjustable delay line 209 which is shown interposed
between the output of modulator 206 and polarization splitter 210.
Nevertheless, delay line 209 is not absolutely necessary. It is
also possible to trim the length of one or the other arm, through
one or two trials, to within a few picoseconds of the correct
length so the apparatus may remain all-waveguide throughout.
The original soliton pulse stream output from the correctly
adjusted multiplexer of FIG. 2 would appear as shown in FIG. 3. The
x and y axes represent intensities of pulses of different
(orthogonal) polarizations. As an example, soliton pulses 301 and
302 have an initial polarization along the axis and a period of 400
ps. Soliton pulses 303 and 304 have an orthogonal (y direction)
polarization, the same period, and are time interleaved with the
first series of pulses. Information is carried in the pulse streams
by virtue of the presence or absence of pulses at the expected or
nominal positions on the time axis. Note that launching the soliton
pulses as in FIG. 3 not only achieves the potential for combined
time and polarization division demultiplexing at the receiving end,
but also virtually eliminates the potential for cross-phase
modulation, and hence virtually eliminates the potential for
interaction during transmission, between the two channels.
An alternative circuit to FIG. 2 is shown in FIG. 1, wherein two
laser sources are shown, oriented to provide two orthogonally
polarized beams; in all other respects, the circuit of FIG. 1
functions in a similar manner to the circuit of FIG. 2, however is
absent the polarization controllers 211 and 212.
The aforementioned prior art reference by Miyamoto et al. teaches
the use of delay lines to time-skew the pulse trains that are to be
multiplexed. For example, the paper discloses using two different
lengths of polarization maintaining fibre in order to create a
suitable delay. Although using different lengths of optical fibre
provides a necessary delay, ensuring that this delicately balanced
network is stable over a range of temperatures is not trivial.
Although the prior art optical circuits to some degree provide
solutions for polarization time-division multiplexing, the '322
patent for example describes a rather complex optical circuit where
polarization controllers are shown to control the polarization
state of the light propagating through the optical fibres.
In contrast, the circuit in accordance with this invention is a
micro-optic circuit that does not rely on the use of polarization
controllers and does not require polarization-maintaining optical
fibre.
Furthermore, an aspect of the instant invention provides a
micro-optic delay element, which utilizes the polarization
difference between two data-streams to be time-multiplexed while
preserving the polarization state of the two orthogonal streams.
Furthermore, the instant invention provides a solution, which is
considerably, more temperature-stable than using two separate
waveguides and independently controlling for any temperature
difference between the two waveguides.
In another aspect of the invention a bulk delay optical circuit is
provided wherein to optical paths are provided wherein light
propagating along the two paths is unguided. The unguided beams
comprising separate bit streams are then combined to provide a
single time-multiplexed data stream. This embodiment obviates the
use of optical fiber as part of the delay element and provides for
a more smaller, more stable, more controllable circuit.
This invention obviates the prior art circuits where blocks
performing functions such as rotating polarizations, monitoring,
providing delay, were connected together using polarization
maintaining fiber. Using these prior art methods of assembly
increased cost, lessened precision, where devices were very
sensitive to external perturbation such as vibration and handling
and to temperature changes. In these prior art embodiments, the
length of each polarization maintaining fiber had to be controlled
to a very high level of precision. Differentical delay was
accomplished by careful control of fiber lengths. In addition there
was a section that was adjustable for fining tuning.
SUMMARY OF THE INVENTION
In accordance with the invention a circuit is provided for
receiving two streams of data and for polarization time-division
multiplexing the two streams of data onto a single waveguide such
that one of the data streams is delayed by a time t.sub.d from the
other data stream, comprising: a first modulator having an input
port and an output port for modulating input light and for
providing a first modulated data stream; a second modulator having
an input port and an output port for modulating input light and for
providing a second modulated data stream; a first lens for
collimating light provided by the first modulator; a second lens
for collimating light provided by second modulator, the first and
second lenses each for providing a substantially collimated
substantially unguided beam of light to at least another component;
a polarization beam splitter/combiner having two input ports at one
end optically coupled to receive the substantially collimated,
substantially unguided beams of light, said polarization beam
splitter/combiner having a combining port at another end for
combining the data streams such that one data stream delayed by a
time t.sub.d from the other data stream, light traversing parallel
paths from the first and second lenses respectively to the
polarization beam splitter combiner being substantially unguided,
so that light traversing at least one of said parallel paths will
have a polarization state which is substantially unchanged.
In accordance with the invention, there is provided a circuit for
receiving two streams of modulated data and for polarization and
time-division multiplexing the two streams onto a single waveguide,
comprising: a polarization rotator for rotating the polarization of
one of the two modulated data streams; and, a birefringent crystal
having at least two input ports disposed at one end to receive the
two modulated data streams having orthogonal polarization states,
the birefringent crystal having an output port disposed at an
opposite end to receive and combine the two modulated data streams
into a single time-interleaved data stream, the birefringent
crystal being of suitable length for providing a path length
difference between each of the at least two input ports and the
output port to provide a required time delay at the output port
between the two data streams.
In accordance with another aspect of the invention, a method of
multiplexing optical signals onto an output port is provided. The
method comprises the steps of: providing two modulated polarized
optical signals having a polarization difference between the two
modulated signals of substantially 90 degrees; passing one of the
two modulated signals along a first path in a birefringent crystal;
passing another of the two modulated signals along a second path
intersecting the first path at the output port of the birefringent
crystal.
In accordance with the invention there is provided a fibreless
optical circuit for receiving two streams of modulated data and for
polarization multiplexing the two data streams onto a single
waveguide, comprising: a modulator module for independently, and in
parallel, modulating optical signals and for providing two data
streams; a birefringent crystal having at least two input ports at
an end thereof disposed to receive the two modulated data streams
having different polarization states, the birefringent crystal
having an output port disposed at an opposite end thereof to
receive and combine the two modulated data streams into a single
multiplexed data stream, the birefringent crystal being of suitable
dimensions to provide time division polarization multiplexing, the
circuit being fibreless such that there is an absence of optical
fibre between the modulator module and the birefringent crystal for
coupling light therebetween.
In a broad aspect of the invention, a circuit is provided for
receiving two streams of data and for and time-division
multiplexing and interleaving the two streams onto a single
waveguide. The circuit comprises: modulation means for providing
first and second modulated data streams in the form of a first beam
and a second beam; and, means optically coupled with the modulation
means for routing and combining the first and second beams in an
unguided manner, such that beams of light launched into said means
are unguided as they propagate therethrough in the absence of
waveguides, said means for routing and combining having at least
two input ports optically coupled to receive the first and second
beams and having an output port to on which to combine the two
beams into a single time-interleaved data stream, the means for
routing and combining for providing an optical path length
difference along two paths between each of the at least two input
ports and the output port for light launched into the at least two
input ports on route to the output port to provide a required time
delay at the output port between the two data streams such that the
data within the data streams is time interleaved having a bit
period Dt.
In accordance with the invention a circuit is provided for
receiving two orthogonally polarized streams of data and for
polarization and time-division multiplexing the two streams onto a
single waveguide. The circuit in accordance with this aspect of the
invention includes polarization dependent means having: first and
second input ports at an end thereof; an output port optically
coupled with the first and second ports; a first unguided optical
path and a second unguided optical path disposed between the first
and second input ports respectively and the output port, for
receiving the two orthogonally polarized data streams and for
carrying said data streams to said output port in an unguided
manner, the first and second unguided paths being of a different
optical path length which differ by an optical path length DnL,
wherein the length dnL, is selected to provide a required relative
time delay between the two data streams as they pass along the
first and second unguided paths from the first and second input
ports to the output port such that the data streams become a single
time-interleaved data stream having a predetermined bit period
Dt.
Conveniently, if a delay is required that exceeds the delay that is
provided by traversing the first and second paths of the
birefringent crystal having different optical lengths, a spacer can
be inserted into each of the signal paths prior to the signals
reaching the birefringent crystal, wherein the spacers are of a
substantially different refractive index. This method is quite
suitable when optically coupling a lithium niobate modulating block
with a rutile crystal, wherein no optical fibres are used except
perhaps coupled to output ports.
In summary, the devices in accordance with this invention are small
and compact and integrated. Yet still further, due to their
compactness are somewhat easier to temperature control than, for
example the prior art circuits shown. Yet still further, and
perhaps more importantly, the optical circuit including the
modulator focusing optics between the modulator and a polarization
beam splitter/combiner do not require any optical fibre for
coupling of light therebetween. Advantageously, by an providing a
relatively unguided light path, polarization controllers or
polarization maintaining fibre is not required. As well by
providing block like elements coupled to one another, i.e. one or
more modulator blocks coupled to rod GRIN lenses, coupled to a
birefringent crystal yields a compact easy to assemble device that
can be conveniently packaged.
BRIEF DESCRIPTION OF THE DRAWINGS
Exemplary embodiments of the invention will now be described in
conjunction with the drawings in which:
FIG. 1 is a prior art schematic diagram of a multiplexing circuit
using 2 laser light sources oriented to yield orthogonal polarized
light;
FIG. 2 illustrates a prior art schematic of a multiplexing
circuit;
FIG. 3 illustrates the pulse field envelopes at the output of the
multiplexor of FIG. 2;
FIG. 4 illustrates a schematic circuit block diagram of an enhanced
TDM multiplexor in accordance with this invention;
FIG. 5 is a schematic block diagram which illustrates a portion of
the circuit used for multiplexing and which illustrates the
operation of that circuit portion;
FIG. 6 is a more detailed embodiment illustrating the circuit of
FIG. 5;
FIG. 7 is an alternative embodiment to the circuit of FIG. 6,
wherein two spacers having different refractive indices are
utilized to achieve a time delay between signals traversing the two
spacers;
FIG. 8 is a schematic block diagram of a preferred embodiment of
the invention showing the modulator module coupled to a rutile
crystal via a pair of substantially quarter pitch collimating GRIN
lenses;
FIG. 9 is a schematic block diagram illustrating a polarization
beam splitter/combiner in the form of a polarization beam splitting
cube;
FIGS. 10a and 10b are block diagrams of a polarization bit
interleaver in accordance with a preferred embodiment of the
invention;
FIG. 11 is a schematic block diagram of an alternative embodiment
of the invention that uses a conventional polarization beam
splitter (PBS)/combiner instead of birefringent crystal for
combining the two data streams into a single interleaved data
stream;
FIG. 12 is a schematic block diagram similar to that shown in FIG.
10, however shown with the half waveplate and tap PBS shown in a
different and preferred order such that the individual channels are
orthogonal in polarization when they are monitored; and,
FIG. 13 is a schematic block diagram of an alternatve embodiment of
the invention which is polarization indepenent, and wherein the PBS
of the previous two embodiments is replaced with a beam splitter
(BS).
DETAILED DESCRIPTION
Turning now to FIG. 4, a substantially integrated micro-optic
circuit is shown having a slab waveguide chip 10 having an end
optically coupled with a laser 12 and having an end optically
coupled with a birefringent crystal 14. At an input end of the
crystal 14, a half waveplate 16 is provided for rotating the
polarization of the light passing therethrough by 90.degree..
The slab waveguide chip is LiNbO.sub.3 having waveguide disposed
therein. The waveguides can be formed by ion implantation or
alternatively by grafting polymer or other such light transmissive
material into the chip. Electrical contacts are disposed about the
waveguides 15a, 15b, 15c . . . and in operation a voltage is be
applied to modulate the signal passing between the contacts.
Variable attenuators are provided at the output for controlling the
amplitude of the modulated signals. Although LiNb0.sub.3 is a
preferred modulator, of course other types of modulators may be
used, for example electro-absorption or GaAs. Aside from the
compactness and temperature stability of the circuitry shown within
the waveguide 10, the operation and interconnection of the
components is substantially similar to the circuitry shown in FIG.
2. Notwithstanding, one major difference between the circuit of the
instant invention, shown in FIG. 4 and the prior art circuits, is
the provision of the birefringent crystal for use as a polarization
combiner and delay line for time-division polarization interleaving
of pulses. One even more significant difference in this circuit and
prior art circuits for time-division polarization multiplexing is
the fibreless nature of the circuit from the modulator module 10 to
the beam splitter/combiner, for example shown here in the form of a
crystal 14. By coupling substantially collimating lenses, for
example, quarter pitch GRIN lenses to the modulator 10, collimated
beams are provided to next elements in sequence and to the crystal
14. Since the substantially collimated beam traverses the glass
spacer and quarter waveplate substantially unguided, its
polarization state is substantially unaltered.
FIG. 5 illustrates a portion of the circuit shown in FIG. 4
depicting the operation of the polarization combining and
multiplexing circuit. This circuit conveniently provides the added
advantage of achieving a predetermined required delay. A stream of
pulses spaced by 25 ps are provided at the input end of each of the
GRIN lenses 50a. Light directed through the bottom GRIN lens is
rotated by 90 degrees by the waveplate 16. As can be seen in
figure, this beam must travel a greater distance to reach GRIN lens
50b, than the beam that follows a straight through path launched
into the upper GRIN lens 50a. This in effect skews the pulses in
time that were launched simultaneously into the two GRIN lenses
such that the orthogonally polarized pulses become combined and
time multiplexed, as shown at the output of the GRIN lens 50b. FIG.
6 (not drawn to scale) illustrates in more detail, dimensions of a
birefringent or rutile crystal that achieves a desired time delay
to provide time multiplexing of these two orthogonally polarized
streams of pulses. The length of the crystal in this exemplary
embodiment is 27 mm, and the with is 5 mm. Of course to some
extent, the size of a crystal that is required is proportional to
it cost. FIG. 7 illustrates yet another embodiment, wherein a
spacer of glass 17 is inserted into the upper optical path, and a
spacer of silicone 18 provides a portion of the lower optical path.
By selecting light transmissive materials such as glass and
silicone that have a substantially different refractive indexes in
the two paths the beams must follow, delays in addition to delay
provided by the birefringent crystal 14 can be enhanced and further
controlled between the two. For example, in FIG. 7, the silicone
spacer 18 shown, has a much higher refractive index than the glass
spacer 17; light traveling through the silicone propagates
therethrough slower than light traveling through a similar length
of glass. Notwithstanding, a birefringent crystal of at least some
minimum proportions is required. In the example shown, the beams
propagating through the birefringent crystal 14 are collimated or
near-collimated and substantially separated at the input end of the
rutile. Thus, the crystal must be of dimensions that will support
two beams, combine them, and provide a suitable required delay even
in the instance that additional delay is provided by the silicone
spacer. However, it can be seen, by comparing FIGS. 6 and 7, that
the overall dimensions of the rutile, required to combine and time
multplex the two pulse streams is substantially lessened in the
embodiment of FIG. 7. Nevertheless, this embodiment requires
suitable antireflection coating between the GRIN lens 50a and the
silicone spacer.
It should be noted that in an alternative example, the polarization
beam splitter/combiner described heretofore, is a rutile crystal,
however, a polarization beam splitting cube could be used instead
of the rutile as is shown in FIG. 9.
FIGS. 10a and 10b illustrate an optical circuit is shown wherein
two modulators 90a and 90b fabricated on a single substrate provide
modulated signals to voltage controlled attenuators (VOAs) 102a and
102b. Modulator 90a is an odd channel modulator and 90b is an even
channel modulator. The voltage controlled attenuators 102a and 102b
are controlled by a feedback signals received from a photodetector
array 103. A rotate and delay (RAD) circuit 104 is directly coupled
to the output of the VOAs which combines the two modulated signals
into a time interleaved single bit-stream signal.
Turning now to FIG. 11, an embodiment of the RAD circuit is shown
directly coupled to a lens array 110 for providing two
substantially collimated beams to the down-stream optical elements.
A prism 105 is disposed between the array 110 and the VOA block.
The prism contains apertures that produce the spatial filtering of
the light out of the lithium niobate modulators 90a and 90b. 2.
This means of spatial filtering the outputs between the guided
section and the unguided section improves the signal to noise ratio
(SNR) of the power monitoring devices The lithium niobate block is
polished at an angle to prevent back reflections.
The lens array 110 is optically and directly coupled to a silicon
spacer 112a and a BaK1 spacer 112b providing two unguided paths for
the two modulated bit streams. The two different materials selected
for these paths provide a relative different optical path length
between the paths due to their different refractive indices. In
other embodiments not shown, additional path routing through
elements in an unguided manner can be utilized to achieve this end
without using materials having different refractive indices. A half
wavelplate 114 is shown adjacent to the spacer 112a for
rotating/retarding the polarization of the bitstream received from
a modulator (not shown). A tap beam splitter 116 is optically
coupled to the .pi./2 wave plate 114 and to an end face of the
spacer 112b for providing a portion of the two bit streams to a
photodiode array 118 and for passing through the remaining portion
of the two bit streams to a PBS/combiner 119. The electrical
signals from this photodiode array monitor proportional to optical
signals detected by the photodiode array 118 are used to control
integrated VOAs 102a and 102b shown in FIG. 10. The polarization
beam splitter/combiner 119 receives the two collimated beams from
the spacers and rotator and combine the two beams in a polarization
dependent manner into a single time interleaved bit stream. Output
imaging lens, in the form of a graded-index (GRIN) lens 120
receives and focuses the collimated beam for coupling to a fiber or
waveguide.
Referring now to FIG. 12 a similar circuit is shown, however the
tap beam splitter 116 is shown sandwiched between an imaging system
in the form of lens array 110 and the .pi./2 wave plate 114 and
spacer 112b. The embodiment of FIG. 11 is preferred to that of FIG.
12 as it rotates the polarization prior to the tap beamsplitter
photodiode array VOA control. This minimizes coherent effects at
the photodiode array 118. In both embodiments the use of a lens
array is preferred as it significantly simplifies the assembly
process.
In embodiments described heretofore, a half-wave plate is shown for
rotating the polarization along one path, however, it is
conceivable to provide orthogonally polarized beams of light to the
modulator, obviating the requirement of a rotator.
Another embodiment of the invention is shown in FIG. 13 similar in
some respects to those of FIGS. 11 and 12, however this embodiment
functions in a polarization independent manner. In this embodiment
the PBS is replaced by a 3 dB beam splitter/combiner and the need
for a halve-wave plate is obviated. By using the non-polarizing
beam splitter there is a 3 dB loss for each channel.
In these three latter embodiments there is a common feature which
provides significant advantages related to cost, ease of assembly
and performance. An optical path length difference between two
separate data paths is provided by using bulk components having a
material difference in refractive index or and/or a physical path
length difference and wherein the beams carrying the separate data
traverse the system comprising the spacers and beam splitters in an
unguided fashion.
Advantageously, the dual modulators 90a and 90b are shown in FIG.
10 fabricated on a single lithium noibate substrate; and, each
modulator is followed by a VOA 102a, 102b on the same substrate. By
using the input imaging system, or lens array 110, there is no need
to align each lens independently. The same advantage is gained by
using a photodiode array such that both photodiodes are aligned
simultaneously.
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